The present invention relates generally to a ceramic component coated with a corrosion inhibiting material and more particularly, to a process for making a silicon-based ceramic component with a rare earth coating.
In the operation of gas and diesel engines that are adapted for utilizing alternative fuels, for example, methanol, ethanol, natural gas, the use of ceramic components, such as glow plugs, turbochargers, and turbine blades, are well known. It is well known that such engine components have a less than desirable service life owing to the harsh environment in the engine due to elevated temperatures.
Particularly, in diesel engines, it is also well known that a glow plug is used to beneficially assist the ignition of the non-autoignitable fuel during start-up as well as during operation. Such glow plugs also have a less than desirable service life owing to the harsh environment in the combustion chamber due to elevated temperatures.
Where the heating portion of a glow plug is formed of a silicon-based ceramic, and more particularly, silicon nitride (Si3N4), the service life of the heating portion of the glow plug is further reduced due to thermal stresses, oxidation and corrosion. The operating longevity of a silicon-based ceramic glow plug is further compromised when it is utilized in a diesel engine that is burning fuel other than diesel fuel.
When a silicon-based ceramic glow plug is utilized to assist in the ignition of non-autoignitable fuels at the elevated temperatures needed to sustain fuel combustion, the silicon-based ceramic undergoes severe corrosion and erosion due in part to the presence of impurities such as sodium, calcium, magnesium and sulfur introduced by the fuel and the lubrication oil. At high temperatures, these impurities react with the normally stable silica (SiO2) film layer on the silicon-based ceramic surface to form compounds, such as sodium sulfate (Na2SO4), having a lower melting temperature than silicon-based ceramic, which are progressively eroded away by fuel and air spray.
Coatings are utilized to increase the corrosion and erosion resistance on engine components utilizing alternative fuels. Deposition of coatings on these engine components, such as glow plugs, are well know in the art and are of various constructions with a multiplicity of different materials. The prior art processes employed either a physical vapor deposition (PVD), a chemical vapor deposition (CVD), or plasma spray process.
These, heretofore, utilized processes had many inherent deficiencies. Among the many deficiencies, in particular, they were expensive and required several steps to form an adherent, uniform coating. An example of such a coating on a glow plug, formed by a deposition process, is found in U.S. Pat. No. 5,578,349, filed Nov. 30, 1995, and issued to Kent A. Koshkarian et al. on Nov. 26, 1996 and assigned to Caterpillar Inc.
It is desirable to provide the surface of a component with a protective coating that is not attacked by the impurities in the combustion environment and, thus, inhibits the corrosion and/or erosion mechanism. It is also desirable that the protective coating have very good adhesion to the component surface. It is further desirable that the protective coating have uniform continuity across the surface of the component to provide uniform corrosion and erosion protection. Finally, it is desirable to utilize a simple, low cost process to form a ceramic component with a protective coating.
The present invention is directed to overcome one or more of the problems as set forth above.
This invention applies to a silicon-based component in a corrosive environment. The silicon-based component has a rare earth silicate coating in the range of about 1.0 microns and 5.0 microns.
In another aspect of the invention, a process is provided for a silicon-based ceramic component, preferably a silicon nitride component, with a corrosion inhibiting coating material. The component is oxidized by heating the component at a temperature greater than 1250 degrees C. The rare earth oxide, which oxidizes over time at high temperature, is originally contained in the silicon-based ceramic component.
Upon heating, the rare earth oxide migrates to the surface of the ceramic component during the oxidation reaction and further reacts with the silica (SiO2) film layer on the silicon-based ceramic component. The rare earth oxide in the rare earth oxide-doped ceramic component and the silica film layer form a rare earth silicate. Thus, the rare earth silicate coating layer is self-formed from the reaction.